Process of drilling wells



United States Patent 3,507,343 PROCESS OF DRILLING WELLS James A. Gill and Marvin O. Stearns, Houston, Tex., as-

signors to National Lead Company, New York, N.Y., a corporation of New Jersey No Drawing. Filed Dec. 21, 1967, Ser. No. 692,278 Int. Cl. E21b 21 04 US. Cl. 17566 6 Claims ABSTRACT OF THE DISCLOSURE The invention provides a means of temporarily increasing the density of a selected portion of the circulating drilling fluid system of a well being drilled by the rotary process. To a selected portion of the drilling fluid there is added a granular high density solid, such as crushed or granulated barite finer than 16 mesh and coarser than 80 mesh, and which may have a specific gravity of 4.2, so as to bring about the desired increase in density. When this has served its purpose, it is removed from the circulating system, as by screening or centrifuging, and the overall density of the circulating fluid remains substantially unaffected by the completed process.

This invention relates to the drilling of wells, such as oil and gas wells, by the rotary process, and more particularly to an improvement wherein the density of selected portions of the circulating drilling fluid may be increased and later decreased again at will.

In drilling wells in search of oil and gas, the method almost universally used is the so-called rotary process, in which a bit is rotated at the bottom of the bore hole by means of a string of hollow drill pipe which extends to the surface. A circulating liquid known as drilling fluid or drilling mud is pumped downwardly through the drill pipe, where it issues through ports in the bit and rises in the annular space between the drill pipe and the walls of the borehole. The drilling fluid is discharged into suitable holding vessels, where it may or may not receive chemical or physical treatment, and is taken up again by pumps and circulated down the drill pipe, this circulation being substantially continuous during drilling. At times the circulation is interrupted, most frequently momentarily when an additional section of drill pipe is added at the top of the string. From time to time the entire string of drill pipe together with the bit is withdrawn from the hole, most often to change the bit, and here again the circulation is interrupted.

The density of the circulating fluid is adjusted to the circumstances at hand, ordinarily by adding suitable quantities of very finely ground minerals such as barite or hematite, which are commonly ground so as to pass a 325 mesh screen and thus have particle diameters of less than inch, and thus less than 1.7 mils. This degree of fineness is used so that the weighting material, as such finely ground minerals are termed, will remain in suspension and not settle out. The circulating fluid generally contains varying amounts of colloidal suspending and circulating agents, which may be colloidal clays, hydrophilic organic colloids, and the like for water-base muds, and organophilic colloids, such as soaps, asphalt, organic-treated clays, and the like for oil base fluids.

In many environments the density of the circulating fluid is a critical matter, and especially so when drilling at great depths. The density must be great enough to keep high pressure formation fluids, such as oil, water, and gas, from intruding into the bore hole during the drilling operation. One the other hand, it must not be so great that the mud is lost to the formation. In many instances, especially when drilling at great depths, difficulty is encountered in maintaining the circulating fluid in a pumpable state because of its considerable burden of suspended weighting material.

Another application of the invention arises from the circumstance that, since drilling rate is an adverse function of the pressure differential between the circulating fluid at the bottom of the hole and the formation, other factors being equal, there are occasions when to improve drilling rate the bottom-hole pressure of the circulating fluid is deliberately reduced to the formation pressure or slightly below. Now, the bottom-hole circulating pressure is made up of two parts, first, the pressure drop of the returning fluid in the annulus and second, the hydrostatic head of the fluid. The contribution of the first to the total can result in a sufiicient total bottom-hole pressure for safety. However, when circulation is suspended under these conditions the hydrostatic head of the fluid is less than the formation pressure which is a potentially unsafe condition so that it becomes desirable to be able to temporarily increase the density of the fluid when circulation is stopped and to decrease it again when circulation and drilling are resumed. The present invention is admirably adapted to accomplish this. Only a portion of the mud in the hole need be increased in density by adding granular weight material thereto.

It also happens that the density of the circulating fluid may have been maintained at the proper value during the period of drilling, and suddenly a temporary increase in density is needed. This may happen if a small pocket of high pressure gas is drilled into, whereupon the bulk density of the mud is temporarily lowered by the gas which it is transporting to the surface. In such a case, if the mud system were increased in density by the addi tion of a substantial amount of weight material, then after this crisis had passed the gas was no longer intruding into the hole, the mud would have too great a density, and difiiculty might be encountered from loss of mud to the formation, which would otherwise not be the case had the mud never been increased in density.

Again, from time to time it is necessary to pull the drill string, that is the entire length of coupled sections of drill pipe, out of the hole to change bits. When this is done, particularly if the mud is relatively thick, as each coupled section of drill pipe is lifted out of the hole and uncoupled from the pipe remaining in the hole, mud will be present in the uncoupled section and will spill onto the floor of the derrick, constituting both an impediment and a hazard. Indeed, this may happen even in coupling new sections of pipe as drilling proceeds. If the mud in the pipe were increased in density temporarily, then by hydrostatic equilibrium, the mud level inside the pipe would be lower than the mud level outside the pipe and it would be possible to pull the pipe in a dry condition, thus avoiding the spillage problem.

The object of the present invention is to provide a method for temporarily increasing the density of selected portions of the circulating fluid during the drilling of a well, without bringing about any substantial and undesired increased in consistency, and further providing a method whereby the temporary increase in density can be quickly and easily removed at will.

Other objects of the invention will appear as the description thereof proceeds.

Generally speaking, and in accordance with illustrative embodiments of our invention, where a well is being drilled by the rotary process wherein a liquid is circu lated throughout the drilling fluid system of the well, and wherein as is ordinarily the case the drilling fluid has approximately the same density throughout the circulating system, we select a portion of the liquid in the system and add to that portion a quantity of granular weighting material, which may be more particularly described as a granular solid having a particle size substantially Within the range of 7 mils to 50 mils, and having a specific gravity of at least 3.5. The amount added is whatever quantity is sufficient to increase the density of the selected portion by some preselected amount, which will depend entirely upon the particular circumstances of the drilling operation. After this has been done, at some sub sequent time the circulating fluid is passed through a mechanical treating means so as to remove the granular solid which had been added in the first step of the process. After this has been done, then the circulating liquid or drilling fluid will have been restored substantially to its original density.

The particle size range of 7 to 50 mils, a mil being a V1000 of an inch, corresponds closely to 80 mesh and 16 mesh respectively on the US. Standard Screen Scale. As already noted, this is far coarser than the particle size of weight material which is added to the mud in the ordinary course of drilling, with the intention that the weighting material remain in the mud.

The mechanical treatment methods are well known in drilling technology. The simplest and most certain is the employment of a vibrating screen, which is standard equipment on drilling wells generally. If the screen has a mesh size of 80, then when the mud is passed therethrough it will remove substantially all of the granular solid from the mud. Separators depending upon centrifugal action may also be used. These comprise centrifuges as the term is commonly used in the chemical processing arts, wherein a vessel which is ordinarily cylindrical or conical-cylindrical is rotated and the liquid passed therethrough; or devices in which a centrifugal action is imparted to the fluid by the fiow itself, as in hydrocyclones. Typical centrifuges are described in the book Chemical Process Machinery by Riegel, New York, 1953, while hydrocyclones are described in the book Cyclones in Industry, edited by Rietema and Verver, Amsterdam, 1961.

Needless to say, whatever separating method is used should be so selected that it removes merely the granular solid which had been previously added, and does not remove particles substantially finer than 80 or 100 mesh, and particularly, particles finer than 200 or 325 mesh, since that would remove the regular weight material from the mud, to the extent that that may be present. Thus, centrifuges may be operated under such conditions that the regular, very finely divided weight material is removed. This is sometimes done in the course of reclaiming mud. However, such a centrifuge would have to be operated at a lower rotational rate, or other well-understood changes in its operation should be made so as to remove only the granular solid.

We prefer barite as the granular solid, since it is widely available and readily crushed to the requisite size range, and moreover is inert and nontoxic. It ordinarily has a density of from 4.0 to 4.5, although ores rich in barite may be crushed to yield a granular solid of a density as low as 3.5, and still be useful in accordance with the invention. A second choice is hematite, which is available in the requisite particle size as a cylinder from the burning of pyrite, or may be readily crushed down to the requisite size. Witherite, celestite, magnetite, galena, and ilmenite are other minerals which are well-adapted for use in accordance with the invention, and mixtures of any of the foregoing may likewise be employed. Crude forms of any of the above find application in the invention, such as the so-called black sands which have a high content of ilmenite.

The increase in density which is temporarily achieved in accordance with the invention may be slight and still enable the benefits of the invention to be obtained. Thus, an increase in mud density of as little as /10 Pound per gallon will be useful in some instances, and may be all that is needed to lower the mud level inside the drill pipe sufficiently to enable the latter to be pulled dry. On the other hand, in certain emergencies it may be desirable to increase the weight of selected portions of the circulating m ud system by as much as 4 to 6 pounds per gallon. This may be readily accomplished in accordance with the invention. The gallon is the standard U.S. gallon, in which system water has a density of 8.33 pounds per gallon.

The invention is not limited to muds of any particular base liquid, and is as readily adapted for use with water base muds as with oil base muds and emulsion muds of all types.

Some examples of the procedure in accordance with the invention will now be given.

EXAMPLE 1 Tests were made on a commercial drilling mud of the inverted emulsion type, that is, the oil was the continuous phase, having a ratio of oil to water of 70:30. It had a density of 16 pounds per gallon, as a result of its content of minus 325 mesh barite. One sample of this m=ud was weighted to 17 pounds per gallon with regular minus 325 mesh barite, while a second sample was weighted likewise to 17 pounds per gallon with granular barite of 30 to 60 mesh (23 mils to 10 mils). Tests for viscosity, yield point, initial gel, and 10 minute gel in accordance with the procedures of the American Petroleum Institute gave the following results:

It will be seen that increasing the weight of the mud by one pound per gallon did not increase the consistency of the mud as much when granular barite was used. The samples were placed in settling columns approximately 2 inches in diameter and 12 inches high and allowed to stand for varying times from /2 hour to 6 hours. The density of the lower thirds of the columns was determined, and it was found that the slight increase in density upon standing 6 hours was substantially the same for both of the 17 pounds per gallon samples. It was concluded that the stability under the stated conditions of the two weighted muds was for all practical purposes equal.

Both of the 17 pounds per gallon mud samples were then screened through a 100 mesh screen. The density of the mud which had been weighted with regular powdered barite was unchanged, While that which had been weighted with the granular barite dropped to about 16.05 pounds per gallon, indicating substantial restoration to its original mud weight.

EXAMPLE 2 In a well being drilled in West Los Angeles, Calif, at depths from 7526 feet to 8029 feet, over a period of eleven days, using a mud weighing 10.0 pounds per gallon, each time a trip was made, that is, each time the entire drill pipe was withdrawn from the hole and subsequently lowered again, 1400 pounds of granulated barite commercially screened finer than 30 mesh and coarser than 60 mesh were added to the mud in the drill pipe. Each time this enabled the pipe to be pulled dry. The density of the portion of mud so treated, when circulated to the surface in the normal course of drilling after the trip was completed, was approximately 10.6 pounds per gallon. The returned portion of mud was passed over a double-decked shaker screen, one screen of which was 30 mesh and the other screen of which was mesh, Which served to remove the added granular barite which was then discarded. Thus, the circulated mud system was maintained at the mud density of 10.0 pounds per gallon which had been specified by the mud engineer in charge; dry trips were achieved at a relatively low cost; and no subsequent dilution of the mud was necessary to lower the weight back to 10.0 pounds per gallon, as

would have been the case had finely ground barite been used instead of the coarse ground barite. During the eleven day period a total of 7 trips were made in this fashion with entirely satisfactory results.

It will be observed that the objects of our invention have been accomplished in accordance with the foregoing. Furthermore, it is to be noted that the invention is a broad one and while we have described it with the aid of numerous specific examples, nevertheless, much variation is possible in the details of procedural steps, components, proportions, and the like without departing from the spirit of the invention as delineated in the claims which follows.

Having described our invention, we claim:

1. In the process of drilling a well wherein a liquid of a particular density and containing particles smaller than about 7 mils is circulated throughout the drilling fluid system of said well, the improvement which comprises:

adding to at least a portion of said liquid an amount of granular solid having a particle size substantially within the range of 7 mils to 50 mils and a specifiic gravity of at least 3.5 in a quantity sufiicient to increase the density of said portion by at least a preselected value;

and thereafter in the course of said circulation removing said added granular solid but without removing said particles smaller than about 7 mils from said liquid whereby said liquid is restored substantially to said particular density.

2. The process of claim 1 wherein said granular solid is removed by screening action.

3. The process in accordance with claim 1 wherein said granular solid is removed by centrifugal action.

4. The process in accordance with claim 1 wherein said granular solid is selected from the group consisting of barite, Witherite, celestite, hematite, magnetite, galena, ilmenite, and mixtures thereof.

5. The process in accordance with claim 1 wherein said portion comprises liquid contained in the drill pipe portion of said system.

6. The process in accordance with claim 1 wherein said preselected value is within the range of about 0.1 to about 6 pounds per gallon.

References Cited UNITED STATES PATENTS 2,156,333 5/1939 Cross l66 2,756,965 7/1956 Howe l7566 2,870,990 1/1959 Bergey l7566 3,399,739 9/1968 Goodwin l7566 X 3,400,819 9/1968 Burdyn l7566 X NILE C. BYERS, 111., Primary Examiner 

